1. Field of the Invention
The present invention relates to a Dedicated Short range Communication (DSRC) controller and a method used for such systems as the Electronic Toll Collection (ETC) system.
2. Description of the related art
The Electronic Toll Collection (ETC) system on a toll road has been operated as a forerunner of the Intelligent Transportation Systems (ITS). In the ETC, a toll is automatically collected by way of wireless communications using the communications control system called the Dedicated Short range Communication (DSRC) between an ETC unit mounted on a vehicle and a roadside wireless apparatus installed in a tollgate.
DSRC modulation systems in the ETC include the Amplitude Shift Keying (ASK) system and the Quadrature Phase Shift Keying (QPSK) system. In some DSRC controllers of the related art car-mounted DSRC communications units, the control section assigns a waiting priority order based on the information such as a receiving frequency band and a receiving field strength and determines that a signal whose reception is finalized earliest (a signal whose unique word 1 is detected) is “received” and selects the signal (for example, refer to Japanese Patent Laid-Open No. 2003-69544).
FIG. 3 is a block diagram showing an exemplary configuration of a DSRC controller in a related art car-mounted DSRC communications unit. In FIG. 3, a numeral 301 represents a QPSK demodulator, 302 a QPSK unique word drawing section, 303 an ASK demodulator, 304 an ASK unique word drawing section, 305 a communications control system switching circuit, and 306 a unique word format storage section.
The DSRC controller comprising a QPSK demodulation system and an ASK demodulation system compares a unique word drawing result with a unique word stored in the format storage section 306 by the communications control system switching circuit 305, and in case any unique word is detected, outputs a synchronization signal. In this way, the DSRC controller identifies the communications system used by a received signal from among the plurality of communications systems and selects a proper system to process the demodulated data.
Some of the DSRC controllers in related art car-mounted DSRC communications units determine which modulation system is used and select a demodulator by observing the transition of the power value of a received signal from an RF section (for example, refer to Japanese Patent Laid-Open No. 2002-368829).
FIG. 4 is a block diagram showing another exemplary configuration of a DSRC controller in a related art car-mounted DSRC communications unit. In FIG. 4, a numeral 401 represents a receiving power measurement section, 402 a modulation system determination section, 403 a selector switch, 404 a QPSK demodulator, and 405 an ASK demodulator. The receiving power measurement section 401 measures the power value of a received wave. The modulation system determination section 402 determines a modulation system by checking, based on the measurement result, whether a state where a power value below a threshold continues for a predetermined period. The selector switch 403 toggles based on the determination result to select the QPSK demodulator 404 or the ASK demodulator 405.
In recent years, with the increase in the ETC or DSRC applications using the DSRC communications system, a plurality of communications areas are overlapping one another. This presents the current problem of jamming in a related art DSRC communications controller. It is thus necessary to determine a mating roadside machine from among the plurality of roadside machines while preventing possible jamming even in a state where a plurality of communications areas overlap one another. More urgent information must be communicated to a car-mounted DSRC communications unit earlier than other information.